The present invention relates to a room temperature-curable organopolysiloxane composition or, more particularly, to an organopolysiloxane composition having two-way curability at room temperature by irradiation with ultraviolet light and under the influence of atmospheric moisture and useful as an adhesive, sealing material and so on.
Various types of room temperature-curable organopolysiloxane compositions capable of giving a rubbery elastomer by curing are known in the prior art. They are classified into two classes depending on the type of the procedure to cause crosslink formation between the organopolysiloxane molecules. The first class of the compositions includes those stable and storable under a hermetically sealed moisture-free condition but curable by the reaction with the atmospheric moisture. The second class of the compositions includes those stable and storable in dark but curable when irradiated with actinic rays or, in particular, with ultraviolet light.
These two classes of the curable organopolysiloxane compositions have their respective problems and disadvantages. A problem in the moisture-curable compositions, for example, is the relatively low curing velocity as a natural consequence of the crosslinking mechanism that the velocity of curing is under a rate-determining factor of the diffusion of the atmospheric moisture penetrating the surface into the core portion of a body of the composition. Such a slow-curing composition is of course not suitable when rapid curing within, for example, a few seconds is desirable or essential and cannot be used as an adhesive or sealing material of which the effects of adhesive bonding or sealing should desirably be exhibited within a short time. The ultraviolet-curable organopolysiloxane compositions, on the other hand, have a disadvantage that, although very rapid curing can be achieved in the portions under direct irradiation with ultraviolet, no complete curing can be obtained excepting the very surface layer when the composition is colored by formulating a pigment or, if not colored, in a shaded portion not under direct ultraviolet irradiation. In particular, thick bodies of the composition can never be fully cured to the core portion where the ultraviolet light cannot reach even though the surface layer has been satisfactorily cured.
An organopolysiloxane composition improved relative to the above mentioned problems has been proposed and disclosed in Japanese Patent Kokai No. 60-231761. The composition proposed there comprises, as the curing agent added to the diorganopolysiloxane terminated at each of the molecular chain ends with a silanolic hydroxy group as the principal ingredient, (1) a combination of a vinyl silane compound having a vinyl group and a hydrolyzable group in the same molecule or a partial hydrolysis product thereof and (2) a mercapto group-containing organopolysiloxane having, in a molecule, at least two mercapto groups. The composition of this type is curable by the ultraviolet-induced addition reaction between the vinyl groups and the mercapto groups when under ultraviolet irradiation while the curing reaction in the portion not reached by the ultraviolet light can proceed by the mechanism of the condensation reaction between the silanol groups of the diorganopolysiloxane and the hydrolyzable groups of the silane compound. The curing behavior of the composition of this type, however, is not quite satisfactory, presumably, due to the relatively small number of the vinyl groups introduced into the composition. This problem of poor curability could be solved by replacing a part of the monovalent hydrocarbon groups in the diorganopolysiloxane with vinyl groups so as to substantially increase the overall number of the vinyl groups in the composition if it were not for the sacrifice in the elongation of the cured composition with an unduly increased hardness.
Accordingly, the inventors have further continued extensive investigations on the organopolysiloxane compositions curable in two ways of the ultraviolet-induced photochemical addition reaction and the condensation reaction in the presence of the atmospheric moisture. A proposal was made on the base of the discovery obtained in the investigations but the composition thus developed was not practicable due to the unduly short tack-free time of only a few minutes or shorter or unduly rapid surface filming to cause inconvenience in the application of the composition.